While melt refining or purification can be effected in a variety of ways, it is known to refine or purify a metal bath, e.g. of steel, by the introduction into the melt of specific solids, including calcium or calcium compounds, ferrosilicon, Perrin slags, and other aluminum-containing compositions, soda and the like. Since these materials must be mixed with the bath, the most common form of introduction was injection in an inert gas. During the reaction process, these approaches have generally led to temperature losses which had to be tolerated or compensated by heating independently of the supply of the purifying reactants to the melt.
Another problem also characterized earlier systems for the purifying of a metal bath utilizing such materials. Generally the bath is overlain by a slag which can interfere with the introduction of the materials into the melt. The removal of the slag is time consuming and requires special apparatus at high capital cost. Even with earlier efforts to remove the slags, it was not always possible to completely eliminate the slag layer.
The problem of the presence of a slag on top of the melt was largely circumvented by a process as described in German Pat. No. 22 53 630. In that process, a portion of the bath was so confined that it possessed a slag-free upper surface zone through which the refining materials could be introduced into contact with the melt.
This confinement was effected by a pipe which was lowered vertically into the melt and had at its lower end a cap which prevented entry of slag into the pipe but was destroyed at the higher melt temperatures after the lower end bearing the cap was below the slag layer. The slag-free volume of the melt within the pipe could be treated with metallurgically effective materials of the type described and the homogenization of the portion with the pipe with the remainder of the bath was effected by introducing an inert gas stream from below into the pipe.
U.S. Pat. No. 3,971,655 describes a similar process as well as a number of variations on the devices used. The pipe element which is employed is so constructed that above the slag-free zone, a nonoxidizing atmosphere can be maintained.
The problem described above has also been attacked by an approach as described in U.S. Pat. No. 4,518,422 which charges a limited bath volume with the refining substances concurrently with the introduction of oxygen in a gaseous form so that an exothermic reacticn with the oxygen can be generated to liberate heat to the melt and give rise to reactive slags such as the aluminum-based Perrin slag. To ensure that the heat is effectively delivered to the bath, the latter must be mixed and this is done by means of an inert gas stream which, in its turn, has a cooling function. A certain amount of the heat is also lost to the bath through the pipe wall.